1. Field of the Invention
The present invention relates to the improvements of an artificial knee joint used in orthopedic treatment to restore knee joints significantly deformed by rheumatism or osteo-arthritis and causing pain and difficulty in walking, or those broken (or damaged) by bone tumor or by traumatic injuries incurred in traffic accidents or during sporting activities.
2. Prior Art
Artificial knee joints have been studied and used practically just recently. Like an artificial hip joint, an artificial knee joint receives flexion and extension movement loads on its joint surfaces while sliding during walking or physical exercise; the artificial knee joint thus functions as a load receiving joint. Since joint problems and breakage are caused frequently by diseases or traumatic injuries, cases of replacing and restoring knee joints using artificial knee joints have been increasing steadily these days. A typical structure of an artificial knee joint is shown in FIG. 1. The joint comprises a femur member A secured to the lower end surface (distal surface) of a femur F and a tibia member B secured to the upper end section (proximal tibia) of a tibia T. The member A integrally comprising a joint front wall al being upright on the front side, a couple of joint condyles a2, a2 that are gradually extended backward in a nearly arced shape to slide on the concave joint surface b2 of the sliding section b1 of a tibia member B, and joint rear walls a3, a3 being upright in an acute arced shape behind the joint condyles a2, a2. The tibia member B comprises the above-mentioned sliding section b1, the concave joint surface b2 formed on the sliding section b1, and an embedding section b3 to be secured in a proximal canal of the tibia T. The distal end of the femur F is cut off and the femur member A is attached to the femur F by using bone cement. The tibia member B is embedded and secured to the upper end (proximal end) of the tibia T. The joint surface b2 of the sliding section b1 slidably receives mainly the joint condyles a2, a2 (when the knee is bent and stretched slightly) or slidably receives both the joint condyles a2, a2 and the joint rear walls a3, a3 (when the knee is bent sharply) to allow the knee to be bent and stretched. FIGS. 2A and 2B are perspective views illustrating the members A and B separated from the knee joint shown in FIG. 1. In the case of the above-mentioned conventional artificial knee joint, the sliding section b1 is made of HDP (high-density polyethylene) to highly slidably receive the member A and needs a seat section b4 (see FIG. 2B) for reinforcement.
Both the members A and B of the conventional artificial knee joint are made of a metal harmless to a living body, such as pure titanium, titanium alloy or cobalt-chromium alloy, or made of a ceramics harmless to a living body, such as alumina ceramics or zirconia ceramics. Such metal and ceramics materials have both their advantages and disadvantages. When made of metal, the seat section b4 can have relatively high resistance against impact stress even if it is thin. When made of ceramics, however, the seat section b4 should be twice as thick as that made of metal to obtain the same impact strength as that of the seal section b4. In view of the amount of wear, the joint surface b2 of the sliding section b1 made of metal wears more significantly when it contacts the femur member A made of metal than when it contacts the femur member A made of ceramics. If the total thickness of the sliding section b1 and the seat section b4 of the tibia member B is made larger, the cutting amount of the tibia T must also be made larger. This is not desirable from the orthopedic surgery point of view. Taking these into consideration, the thickness of the sliding section b1 is set to 8 mm. A first problem to be solved is how to increase the service life of the tibia member B while the thickness of the sliding section B1 is limited to 8 mm. The femur member A made of metal is combined with the tibia member B made of metal, or the femur member A made of ceramics is combined with the tibia member B made of ceramics. Each combination has its advantages and disadvantages. In particular, the femur member A made of ceramics is inherently weak in resistance against a dynamic load transmitted from the femur F mainly in the vertical direction, that is, external force exerted to the internal sections of the joint condyles a2, a2 of the femur member A to slide the femur member A on the sliding section b1. To solve this problem, a pair of reinforcing ribs a4 with nearly horizontal upper edges are formed in parallel as shown in FIGS. 2A and 3. The strength of the femur member A made of ceramics against the above-mentioned external force can be approximated to the value obtained in the bending strength test shown in FIG. 4. The test is conducted in the manner described below. The femur member A is placed upside down on a support D and secured to it by using cement C. Load E is gradually applied to the member A from above to exert force so that the member A is bent outward (forward and backward). The bending strength of the member is obtained by measuring the load applied at the point the internal surface of the member A is cracked or broken. In this kind of test, cracks and breakage are apt to appear near a connection section all between the reinforcing rib a4 and the joint front wall al. This means that bending stress is concentrated at the portion. When the knee joint is bent sharply, bending stress is concentrated at the connection section a31 located on the rear side of the rib a4. In this case, the joint rear wall a3 slides on the joint surface b2 to receive the external bending force. If any crack or breakage occurs inside the femur member A of the artificial knee joint as described above, the joint may not function or bone breakage may occur, resulting in a serious danger.
This is a second problem to be solved; bone breakage accidents and obstacles against smooth joint movement need to be eliminated by increasing the bending strength of the femur member.